Computer-Based Guidelines for Concrete Pavements Volume II

CHAPTER 3. EARLY-AGE PAVEMENT DISTRESSES

3.1 PLASTIC SHRINKAGE CRACKING

Plastic shrinkage
cracking is an early-age pavement distress that forms before the set of a
freshly placed concrete pavement. Plastic shrinkage cracks are the short irregular cracks that form on the
fresh surfaces of concrete (see figure 18). They can be from few centimeters to just under 1 m long. The crack spacing is irregular, varying from
a few centimeters to 0.6 m apart. Plastic shrinkage cracking is caused by the rapid loss of water from the
surfaces of the fresh concrete. The
cracks form when the rate of evaporation is greater than the concrete's
bleeding rate. According to experience
and previous research, conditions where the evaporation rate of a pan of water
in excess of 1.0 kg/m2/hr will cause cracks to form in concrete of
the same temperature.(6) Caution should be exercised when the evaporation rate exceeds 0.5 kg/m2/hr.

Figure
18. Plastic shrinkage cracking in concrete
pavement.

With the loss of
water from the pavement surface, there is a volumetric contraction of the fresh
concrete. The shrinkage occurs
primarily in the paste, with the aggregate acting only as restraint. These differential volume changes can induce
tensile stresses in the pavement, and can subsequently cause cracks to
form. The fresh concrete does not have
sufficient strength to resist these capillary stresses within the fresh
paste. There is currently no way to
predict with certainty when plastic shrinkage cracks will form.

The method
currently used to predict evaporation rate was developed by C. Menzel in
1954. It uses air temperature, relative
humidity, concrete temperature, and wind velocity to determine if the
evaporation rate is high enough for plastic shrinkage cracks to form. When the rate of evaporation is 0.5 kg/m2/hr,
cracks can occur. When the rate exceeds
1.0 kg/m2/hr, precautionary measures are mandatory. This procedure originally was developed
based on the rate of water evaporated from a standing pan of water.

Evaporation of
water from the surface of freshly placed concrete is primarily due to climatic
conditions. Typically, plastic
shrinkage cracking occurs when construction takes place in hot weather. The climatic conditions that are suitable
for their formation are:

High concrete temperatures.

Low ambient relative humidity.

High wind velocities.

These three
conditions increase the rate of evaporation from the surface of the concrete
pavement. Water also can be extracted
from the slab by absorption or suction into the subbase and/or formwork. This can aggravate the water loss and promote
additional cracking.

Plastic shrinkage
cracks do not always form during the hot weather months. Other factors in addition to climate can
influence the behavior of the fresh concrete. Some cracks are caused by incorporating new materials into the concrete,
such as excessive fines, admixtures, and fiber reinforcement. Fines have a greater water demand and can
affect the bleed water rate. Admixtures, such as superplasticizers and retarders, also affect the
plastic state of the concrete by making the concrete experience less bleeding,
and delay the set. Polypropylene fibers
have been effective in delaying plastic shrinkage cracking. Similarly, the size of the concrete structure
influences plastic shrinkage, since the slab depth is related to bleed rate.(23) These
factors influence plastic shrinkage cracking, and the method to predict crack
formation should account for these different material constituents and size

The
loss of water from the surface of the concrete must be minimized to prevent
plastic shrinkage cracking. One option
is to moist cure the surface of the concrete immediately after placement, and
to continue to do so for at least 24 hours. The most effective method is to keep the surface of the pavement
wet. Other options are to erect wind
barriers around the pavement or to erect sunshades to protect the surface from
heat.

Another
ongoing FHWA project aims to develop a Microsoft® Pocket personal
computer (PC)-based system with guidelines on curing of PCCP using concepts in
the FHWA curing guidelines study and in HIPERPAV II. In addition to guidelines on selecting, applying, and timing curing
methods, the Pocket PC system will have the capability to monitor real-time
concrete temperature for determining concrete maturity and predicting concrete
strength, among other features.

3.2 CRACKING DUE TO THERMAL SHOCK

Placement of
concrete pavements during hot weather conditions when the temperature of the
air exceeds 32 °C, may be undesirable with respect to pavement behavior. During hot weather concreting, the cement
hydration is accelerated by the temperature of the air and the initial high
temperature of the mix components. Depending on the cement composition, cement fineness, and admixtures
used, the accelerated cement hydration may result in significantly higher heat
development during the first hours after placement. This increased hydration also reduces the set time and
complicates the paving operations, delaying the time for proper curing. The higher heat development in the concrete
mix increases the loss of moisture in the concrete, increasing drying
shrinkage. Undesirable hot weather conditions
can be compounded further with the use of high heat cements, high cement
contents, and certain admixtures. In
addition, drastic temperature drops during the first days after concrete
placement may significantly increase the tensile stresses in the pavement. If precautions are not taken to minimize the
above situation, excessive stresses in the concrete pavement may develop that
can result in what is commonly known as thermal shock, or random, uncontrolled
cracking.

Although the
strength of the concrete develops faster due to the higher hydration rate, the
long-term strength is usually lower than that of concrete hydrating at a lower
temperature. After the first 72-hour
period, it has been found that the early-age effects (accelerated hydration and
rapid strength gain) become minimal.

3.2.1 Distress Manifestation on JPCP

For the case of
JPCP, thermal shock may occur in the form of random cracking before, or even
after, the time when joints are sawed. Although these cracks may be tight initially, they may extend to full
depth, affecting the structural integrity of the pavement. Traffic loads and subsequent temperature
fluctuations usually will increase the extent and deterioration of the
pavement, providing poor performance in the long term.

3.2.2 Distress Manifestation on CRCP

As figure 19
illustrates, thermal shock may be observed in CRCP in the form of very closely
spaced cracks. In addition, the cracks
tend to meander more than cracks developed during placements at lower
temperatures. Also, cracks occurring
during the first few hours tend to be wider than those occurring at later
ages. The formation of longitudinal
cracks is another typical distress associated with high temperature placement
(see figure 20). According to
experience, closely spaced transverse cracks and longitudinal cracks due to
thermal shock are more prone to develop into spalling and punchout distresses
in the long term as a consequence of traffic loadings and climate.

3.2.3 Recommended Precautions against Thermal Shock

When significant
changes in temperature are expected during the construction of concrete
pavements, it is important to assess the risk of damage to the pavement, as
well as measures that would keep the stresses in the concrete at an acceptable
level. Alternatives such as modifying
the temperature of the mix or curing methods to insulate the pavement from the
environment may be used to control the pavement temperature and excessive
moisture loss.

The initial
temperature of the mix can be reduced in several ways, such as cooling down the
mixing water, sprinkling or fog spraying the aggregates, or maintaining the
aggregates in shade storage. In
addition, minimizing the concrete hauling time will reduce the time that the
concrete mix is exposed to hot weather before placement. Scheduling placing operations during times
of the day, when climatic conditions are not as critical, will also minimize
the risk of developing extremely high temperatures in the concrete mix.

Protecting concrete
against moisture loss during the curing period to avoid excessive drying
shrinkage can be accomplished by increasing the application rate of the curing
compound, or by using curing methods that provide moisture insulation such as
polyethylene sheeting. If drastic
temperature drops are expected, a combination of curing methods such as
polyethylene sheeting and cotton mats may be necessary to keep moisture in the
concrete, and to provide a more uniform curing temperature. If curing procedures are not performed on
time, excessive moisture loss in the pavement may not be avoided.

Recommended
precautions to avoid thermal shock could also include the use of low heat
cements and supplementary cementitious materials (SCM) such as fly ash. Retarding admixtures also may be used to
help minimize the water demand during hot weather concreting. Some retarding admixtures possess water-reducing
and set-retarding properties. Other
types of admixtures may help prevent drying of the surface by increasing early
bleeding. However, caution must be
taken that the admixtures do not reduce the tensile strength or tensile strain
capacity of the concrete.(25)

Figure
20. Longitudinal crack in CRCP due to thermal
shock. Crack is enhanced for clarity.

An important factor
that can determine the potential for early-age cracking in newly constructed
JPCP is the timing of the joint sawing operations. The purpose of joints in a jointed concrete pavement is to
control the location of the cracking that will naturally occur during the life
of the pavement. The majority of this
cracking will occur during the early-age period as a result of restraint to
volumetric changes. If the joints are
not sawed early enough, uncontrolled cracking can result. This may lead to undesirable long-term
performance due to poor load transfer and spalling. Therefore, sawcutting operations should be performed as soon as
possible following construction to
minimize the potential for uncontrolled cracking. However, when sawcutting begins is also constrained by the time
required to gain sufficient PCC strength to support the weight of the equipment
(and operator), as well as the forces introduced by the cutting blade during
the cutting operations.